1. Field of the Invention
The present invention relates generally to an improved data access arrangement interfacing a modem with a telephone line, and specifically to a data access arrangement having a combined circuit configuration for performing both remote hang-up detection and ring detection.
2. Description of Related Art
In order to transmit digital data between computers over the public switched telephone network (PSTN), modems are used to convert between a computer's digital signals and analog signals that can be carried on the telephone network's analog transmission lines. The subscriber portion of the telephone network (PSTN) has two wires known as "tip" and "ring," which carry information being transferred to and from the subscribers, as well as control signals, such as a ring signal. The modem receives a serial stream of bits as input from a computer and produces a modulated carrier as output, thus converting the digital signals of the computer to analog signals for transmission on the telephone wires. Because modems connect to these conventional telephone wires, they must conform to the requirements of the telephone network (PSTN). A data access arrangement (DAA) is connected as an interface between the modem and the telephone network (PSTN) to ensure the modem is compatible with the telephone network (PSTN).
Telephone companies require that a DAA be placed in series between the telephone wires and all equipment, such as modems, connected to the telephone network (PSTN) in order to isolate the equipment from the telephone network (PSTN). A DAA typically provides this electrical isolation between modems and the telephone network (PSTN), as well as providing impedance matching, hybrid circuit and sometimes amplification, filtering and control functions. DAAs commonly utilize transformers or optical couplers as safety components to provide such electrical isolation.
All equipment connected to the telephone network (PSTN) is either in an On Hook mode or Off Hook mode. During the On Hook phase, the equipment is not connected to other remote equipment through the telephone network (PSTN), and the equipment is awaiting a ring signal from a central office of the telephone network (PSTN). The DAA includes a ring detection circuit in order to detect when a ring signal is transmitted along the telephone wires, where the DAA then notifies the connected modem of the detected ring signal. During the Off Hook phase, the equipment is connected to the central office of the telephone network (PSTN) for the purpose of establishing a connection with other remote equipment. A DAA may also include a DC detection circuit which monitors the DC current on the telephone line to detect when a remote device discontinues its connection with the telephone line (e.g., hangs-up). The DAA then informs the connected modem of the remote device hang-up, so that the connected modem knows that the previous connection has been terminated and the modem can now establish a connection with another remote device. Since both the ring detection circuit and the DC detection circuit provide a connection between the modem and the telephone network (PSTN), safety barriers, such as transformers or optical couplers, must be respectively positioned in series between each circuit and the modem to provide electrical isolation for all electrical pathways between the telephone network (PSTN) and the modem. Thus, current DAAs each include two safety barriers, one for the ring detection circuit and another one for the DC detection circuit. In manufacturing DAAs, approximately 50% of the cost of producing a DAA comes from the cost of the safety barriers themselves.
There is a need for an improved DAA which utilizes a single safety barrier for providing electrical isolation between a modem and the telephone network (PSTN) for signals transmitted to the modem indicating both ring detection and remote hang-up detection in order to significantly reduce the cost, complexity, and size of the DAA.